std.path.setExt

[Mike via Digitalmars-d]

`setExtensionLazy`

Rationale
* sorts well with `setExtension` for tooling
* implies that it is related to `setExtension` in some way
* at-a-glance disambiguates it from `setExtension` with a clue as 
to how it is differrent

The disadvantage is that its a little long, but that's due to the 
length of "Extension" not the "Lazy" suffix.  If it's a problem 
simply create `setExt` and `setExtLazy` and have `setExtension` 
forward to `setExt`for backward compatibility with a friendly 
comment.  It wouldn't even need to be deprecated.

Mike